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A long-term (17-year duration) field experiment was started on newly cleared zinc (Zn)-deficient sandy duplex soil (sand with lateritic ironstone gravel over clay) in south-western Australia that had never been fertilised to measure the residual value of Zn oxide for growing spring wheat (Triticum aestivum L.). When wheat was grown in the field experiment different amounts of Zn were applied once only to plots in 1983, 1984, 1986, 1990, 1992, 1996, 1997, and 2000. When the field experiment was terminated, we collected soil samples from the top 0.10�m of the field plots to do a glasshouse experiment reported here. In the pot experiment 5 amounts of Zn were applied to subsamples of soil collected from each plot of the field experiment. When a yield increase (response) to the freshly applied Zn was obtained in the pot study the maximum yield plateau for the relationship between yield of 54-day-old dried wheat shoots and the amount of Zn freshly applied was similar regardless of when and how much Zn was applied in the field. The amount of Zn freshly applied to soil in the pots required to produce 90% of the maximum yield of dried wheat shoots was determined for soil previously treated with no Zn or different amounts of Zn in different years in the field to provide DM90 values. The DM90 values were largest when no Zn was applied in the field, decreased as more Zn was applied in the field and, for each amount of Zn applied in the field, increased as the number of years from application of Zn increased. For soil treated with 0.5 and 1.0�kg�Zn/ha in the field the relationship between DM90 values and the number of years since Zn was applied in the field was approximately linear. The projected number of years taken for soil previously treated with Zn in the field to require the same DM90 values as soil samples collected from the nil-Zn treatments in the field was 23 and 40 years for the 0.5 and 1.0�kg�Zn/ha treatments, respectively. In the pot experiment the Zn concentration in plant parts that was related to 90% of the total yield of dried wheat shoots (critical Zn concentration) was ~12�mg/kg for youngest mature growth (YMG) and 22�mg/kg for rest of shoots (ROS). The relationship between yield of dried wheat shoots and DTPA soil test Zn before sowing was similar for all the Zn treatments applied in both the field and pot studies, and the critical value was ~0.17�mg�Zn/kg soil. |
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